Recently, there has been a demand for communication systems with improved efficiency. For example, linearity and high efficiency are desired for power amplifiers that are used in radio communication systems. Recent communication systems using multi-value digital modulation often use signals each having an average value of a signal amplitude that greatly differs from a maximum value of the signal amplitude. When such signals are amplified by a normal power amplifier, it is necessary to set an operating point of the amplifier so that the signals can be amplified to a maximum amplitude without a distortion. Therefore, as there is almost no time for the power amplifier to operate near a saturated output at which the power amplifier can relatively maintain to be highly efficient, the power amplifier is used with low efficiency.
In order to solve this problem, various techniques for improving the efficiency of the power amplifier while maintaining linearity have been suggested. One of these techniques is Doherty amplifiers. The Doherty amplifiers aim to improve the efficiency of the power amplifiers, combine outputs of carrier amplifiers and peak amplifiers having operation classes different from each other, and output the combined outputs.
When an input signal level is high, the Doherty amplifier amplifies power using both the carrier amplifier and peak amplifier. On the other hand, when the input signal level is low, the peak amplifier does not operate, and the power is amplified by only the carrier amplifier. Then, even when a large back-off is provided to operate the Doherty amplifier, the Doherty amplifier can be operated highly efficiently. A back-off is a difference between average output power and saturated power. Having a large back-off indicates a state in which the average output power is smaller than the saturated power.
A combining circuit that combines the outputs of the carrier amplifier and the peak amplifier includes a transformer and an impedance converter. When the combining circuit processes signals such as microwave bands, the combining circuit often includes a ¼ wavelength transmission line. In order to have a Doherty amplifier achieve an ideal operation, when a characteristic impedance of the transmission line is Z0, an impedance of a load viewed from a combined point of the outputs is usually set to Z0/2. Usually, Z0 is set to 50 Ω in a high frequency circuit.
As described above, an ideal impedance of the load viewed from a Doherty amplifier in general is not the characteristic impedance Z0 of a system but instead Z0/2, which is half of Z0. Accordingly, when a plurality of Doherty amplifiers are operated in parallel, a circuit that transforms an impedance Z0 of a combiner for combining outputs of the plurality of Doherty amplifiers into a load impedance Z0/2 of the Doherty amplifiers is necessary in an output.
An RF power amplifier that amplifies RF signals with improved efficiency over a wide range of power has been suggested as an example of the Doherty amplifier (Patent Literature 1). FIG. 8 is a block diagram showing a configuration of an RF power amplifier 400 which is an example of the Doherty amplifier. The RF power amplifier 400 includes a carrier amplifier 420 and three peak amplifiers 421, 422, and 423. The peak amplifiers 421, 422, and 423 are connected to an output load 428 via 90° transformers 424, 425, and 426, respectively. The 90° transformer 430 connects a four-way splitter 432 to the carrier amplifier 420. By setting DC biases of the respective peak amplifiers 421, 422, and 423 to appropriate values, Doherty functions can be expanded by the plurality of peak amplifiers. An increase by the plurality of peak amplifiers corresponding to 6 dB can be expected in the power range, and peak efficiency can be maintained for the increased power range. The efficiency is reduced to some extent due to a limited loss of an N-way splitter. A four-way amplifier expands an efficient power range to a theoretical value of 18 dB. As mentioned above, such an expansion is extremely important in digital communication systems that use modulation schemes in which a ratio between a peak and average power is as high as 13 dB. A four-way configuration provides an overall power increase of 3 dBm as compared to a conventional two-way Doherty circuit. Accordingly, an amplifier which has 120-watt-peak output can be provided by respective paths (carrier and three peak amplifiers) that each use a 30 watt transistor in the four-way Doherty configuration.
There is a report of another example regarding peak amplifiers of a Doherty amplifier in which a plurality of amplifiers are arranged in parallel (Non-Patent Literature 1).
Patent Literature 1: Japanese Unexamined Patent Application Publication No. 2010-50986
Non-Patent Literature 1: Junghwan Moon et al., “Highly Efficient Three-Way Saturated Doherty Amplifier With Digital Feedback Predistortion”, IEEE, August 2008, IEEE MICROWAVE AND WIRELESS COMPONENTS LETTERS, VOL. 18, NO. 8, pp. 539-541